Add Testing Codable blogpost

mmatoszko · mmatoszko · commit 00d09b9d3cda · 2019-05-10T22:25:22.000+02:00
diff --git a/source/_posts/2019-05-10-testing-codable.markdown b/source/_posts/2019-05-10-testing-codable.markdown
@@ -0,0 +1,78 @@
+---
+layout: post
+author: matt
+title: "Testing Codable"
+date: 2019-05-10 19:55:56 +0200
+comments: true
+categories: 
+---
+
+Codable is a great protocol available in Swift. It makes it possible to create a type safe JSON representations of structures used within our application with zero boilerplate.
+
+```swift
+struct Person: Codable {
+    let name: String
+    let email: String
+    let age: Int
+}
+```
+
+Once the structure conforms to `Codable` everything works out of the box. There's a nice way to test those structures and make sure that everything gets the exact JSON format that we aligned with backend.
+<!--more-->
+
+Let's create the following protocol in the test bundle:
+```swift
+protocol JSONTestable {
+    init?(_ json: String)
+    func json() -> String?
+}
+```
+
+After that we can immediately provide an extension which conforms to that protocol in the test bundle:
+```swift
+extension JSONTestable where Self: Codable {
+    init?(_ json: String) {
+        guard
+            let data = json.data(using: .utf8),
+            let decoded = try? JSONDecoder().decode(Self.self, from: data)
+            else { return nil }
+        self = decoded
+    }
+
+    func json() -> String? {
+        guard let data = try? JSONEncoder().encode(self) else { return nil }
+        return String(data: data, encoding: .utf8)
+    }
+}
+```
+
+Then there's only one simple step which needs to be done in the test bundle in order to test the structure:
+`PersonTests.swift`:
+```swift
+extension Person: JSONTestable {}
+```
+
+That's it! Now we can easily test the result of the serialization and deserialization :)
+
+Example tests:
+```swift
+final class PersonTests: XCTestCase {
+    func testJsonSerialization() {
+        let person = Person(name: "John Appleased", email: "john@appleased.com", age: 30)
+        XCTAssertEqual(personJson, person.json()) // ✅
+    }
+    
+    func testJsonDeserialization() {
+        let person = Person(name: "John Appleased", email: "john@appleased.com", age: 30)
+        XCTAssertEqual(person, Person(personJson)) // ✅
+    }
+}
+
+private let personJson = """
+{"name":"John Appleased","email":"john@appleased.com","age":30}
+"""
+
+extension Person: JSONTestable, Equatable {}
+```
+
+What do you think about this method? Is there anything which could be improved in this implementation? Please let us know :)
